Aix Marseille Univ., UMR CNRS 7325, CINaM , 13009 Marseille Cedex 09, France.
Electron Microscopy for Materials Research (EMAT), University of Antwerp , Groenenborgerlaan 171, 2020 Antwerp, Belgium.
ACS Appl Mater Interfaces. 2018 Jan 31;10(4):3874-3884. doi: 10.1021/acsami.7b17021. Epub 2018 Jan 22.
The thermal stability of printed polymer solar cells at elevated temperatures needs to be improved to achieve high-throughput fabrication including annealing steps as well as long-term stability. During device processing, thermal annealing impacts both the organic photoactive layer, and the two interfacial layers make detailed studies of degradation mechanism delicate. A recently identified thermally stable poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]:[6,6]-phenyl-C-butyric acid methyl ester (PTB7:PCBM) blend as photoactive layer in combination with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate as hole extraction layer is used here to focus on the impact of electron extraction layer (EEL) on the thermal stability of solar cells. Solar cells processed with densely packed ZnO nanoparticle layers still show 92% of the initial efficiency after constant annealing during 1 day at 140 °C, whereas partially covering ZnO layers as well as an evaporated calcium layer leads to performance losses of up to 30%. This demonstrates that the nature and morphology of EELs highly influence the thermal stability of the device. We extend our study to thermally unstable PTB7:[6,6]-phenyl-C-butyric acid methyl ester (PCBM) blends to highlight the impact of ZnO on the device degradation during annealing. Importantly, only 12% loss in photocurrent density is observed after annealing at 140 °C during 1 day when using closely packed ZnO. This is in stark contrast to literature and addressed here to the use of a stable double-sided confinement during thermal annealing. The underlying mechanism of the inhibition of photocurrent losses is revealed by electron microscopy imaging and spatially resolved spectroscopy. We found that the double-sided confinement suppresses extensive fullerene diffusion during the annealing step, but with still an increase in size and distance of the enriched donor and acceptor domains inside the photoactive layer by an average factor of 5. The later result in combination with comparably small photocurrent density losses indicates the existence of an efficient transport of minority charge carriers inside the donor and acceptor enriched phases in PTB7:PCBM blends.
为了实现包括退火步骤在内的高通量制造以及长期稳定性，需要提高高温下打印聚合物太阳能电池的热稳定性。在器件处理过程中，热退火会影响有机光活性层，并且两个界面层使降解机制的详细研究变得复杂。最近发现的一种热稳定的聚[[4,8-双[(2-乙基己基)氧基]苯并[1,2-b:4,5-b']二噻吩-2,6-二基][3-氟-2-[(2-乙基己基)羰基]噻吩[3,4-b]噻吩二基]]:[6,6]-苯基-C-丁酸甲酯（PTB7:PCBM）共混物作为光活性层，与聚（3,4-乙撑二氧噻吩）聚苯乙烯磺酸盐作为空穴萃取层结合使用，重点研究电子萃取层（EEL）对太阳能电池热稳定性的影响。用致密的 ZnO 纳米粒子层处理的太阳能电池在 140°C 下连续退火 1 天后仍保持初始效率的 92%，而部分覆盖 ZnO 层和蒸发钙层会导致性能损失高达 30%。这表明 EEL 的性质和形态对器件的热稳定性有很大影响。我们将研究扩展到热不稳定的 PTB7：[6,6]-苯基-C-丁酸甲酯（PCBM）共混物，以突出 ZnO 在退火过程中对器件降解的影响。重要的是，当使用紧密堆积的 ZnO 时，在 140°C 下退火 1 天后，光电流密度仅损失 12%。这与文献形成鲜明对比，并在这里解决了在热退火过程中使用稳定的双面限制的问题。电子显微镜成像和空间分辨光谱揭示了抑制光电流损失的潜在机制。我们发现，双面限制抑制了富勒烯在退火过程中的广泛扩散，但仍使光活性层内的施主和受主域的尺寸和距离增加了平均 5 倍。后者的结果与比较小的光电流密度损失相结合，表明在 PTB7：PCBM 共混物中，少数载流子在施主和受主富相内的有效传输。
